1) Field of the Invention
This invention relates to a developing apparatus suitable for use with a printing apparatus of the electrostatic developing type wherein, for example, two-component developer produced by mixing toner powder and magnetic powder is used to develop an image on a photosensitive image carrier.
2) Description of the Related Art
In a printing apparatus which employs electrostatic photography, a photosensitive image carrier (hereinafter referred to as photosensitive drum) in a charged state is first exposed to light of a printing pattern to form an electrostatic latent image on an outer circumferential face of the photosensitive drum. Meanwhile, in a developing apparatus of the printing apparatus, two-component developer produced by mixing toner powder and magnetic powder (carrier) is supplied to an outer circumferential face of a developing roller to form a layer of the developer with a suitable thickness on the outer circumferential face of the developing roller while the developing roller is rotated in contact with the outer circumferential face of the photosensitive drum to develop the electrostatic latent image formed on the photosensitive drum with the developer.
Then, the toner powder image formed on the outer circumferential face of the photosensitive drum in this manner is transferred to printing paper. Thereafter, the non-fixed toner image on the printing paper is fixed by means of a fixing unit to effect desired printing.
In the developing apparatus, the amount of toner powder consumed by printing is calculated from a result of detection of a toner density sensor, a printing density sensor or a like sensor, and an amount of toner powder corresponding to the amount of consumed toner powder is supplemented into the developing apparatus from a toner hopper.
The developing apparatus usually includes, for example, a plurality of agitating and transporting screws, agitation paddles or like members disposed at a position below a toner supplementing port of the toner hopper. The agitating and transporting screws or like members extend in parallel to and are positioned adjacent to each other and rotated to agitate and mix the toner powder and the carrier to produce developer and transport the developer.
The developing roller includes a fixed magnet in the form of a shaft having a plurality of transport magnetic poles, and a rotary sleeve in the form of a hollow tube made of a non-magnetic material such as an aluminum alloy and fitted for rotation around the fixed magnet. The developer can be attracted to an outer circumferential face of the rotary sleeve and transported by the rotary sleeve when the rotary sleeve is driven to rotate.
The developing roller is disposed in the proximity of and in parallel to the photosensitive drum and receives the agitated and mixed developer from the adjacent developer transporting rollers, agitating and transporting screws or like members on the upstream side with respect to the developing region in which the photosensitive drum is developed by the developing roller.
Further, a control member is disposed in parallel to the developing roller over an overall extent of the developing roller on the upstream side with respect to the developing region of the photosensitive drum by the developing roller but on the downstream side with respect to the developer receiving position of the developing roller with a predetermined gap left between the control member and the developing roller. The control member acts to control the amount of developer on the outer circumferential face of the developing roller to a fixed amount and rectify the developer.
In the developing apparatus having the construction described above, toner powder supplemented from the toner hopper is agitated and mixed with carrier and transported as two-component developer by the agitating and transporting screws, agitating paddles or like members. The development is delivered to the developing roller directly or through the developer transporting roller.
The developer delivered to the developing roller is transported by the sleeve of the developing roller being rotated and then is controlled by the control member so that the amount of the developer on the outer circumferential face of the developing roller (the thickness of the developer) may be fixed. Then, only the toner powder in the developer sticks to an electrostatic latent image of the photosensitive drum, thereby to develop the electrostatic latent image on the photosensitive drum.
On the other hand, the carrier in the developer is transported into the developing apparatus again while it is kept attracted to the sleeve of the developing roller also after the development of the photosensitive drum. Then, the carrier is released from the developing roller in the developing apparatus and then collected. The collected carrier is agitated and mixed again with toner powder supplemented from the developing roller by the agitating and transporting screws or the like.
It is to be noted that, in order to facilitate agitation and mixture by the agitating and mixing screws or the like, a technique of shaping particles of toner into a rounded shape is adopted popularly. Further, in order to prevent aggregation of toner powder, a technique of adding an additive to toner powder is employed popularly.
In the developing apparatus described above, the amount of toner powder consumed by printing is calculated from a result of detection of a toner density sensor, a printing density sensor or a like sensor, and an amount of toner powder corresponding to the amount of consumed toner powder is supplemented into the developing apparatus from the toner hopper. Then, in the developing apparatus, the supplemented toner powder is agitated and mixed with carrier using the agitating and transporting screws or like members to produce developer, and the developer is used for development. Thus, in such printing which involves consumption of a large amount of toner powder as continuous printing or printing of a high printing ratio, a large amount of toner powder is supplemented into the developing apparatus from the toner hopper. However, since toner powder has a comparatively small particle size (for example, approximately 6 to 10 .mu.m), toner powder is liable to be aggregated if a large amount of toner powder is supplemented at a time into the developing apparatus from the toner hopper.
Particularly in a high speed printing apparatus, it is required that agitation and mixing of toner powder and carrier be performed in a short time. However, since the load to the agitation driving system including the agitating and transporting screws and so forth and to the developer increases as the agitation speed by the agitating and transporting screws or the like increases, the agitation speed cannot be raised very much. As a result, developer is transported in a locally aggregated state to the developing roller before it is agitated and mixed sufficiently. If such aggregated toner powder is transferred to printing paper from the photosensitive drum, then a resulting print does not exhibit a good print quality.